Efforts are on-going to develop and standardize communications networks and protocols intended to meet the requirements set out for the fifth generation (5G) of wireless systems, as defined by the Next Generation Mobile Networks Alliance. Such networks are expected to support a large number of use cases, with different use cases having widely different requirements in terms of the service provided by the network.
For example, some use cases may involve the remote control of machinery, or surgical instruments. In such cases, it is important that data transmitted between the controller (e.g. a surgeon) and the controlled device (e.g. surgical instruments) is reliable and has low latency. A class of communications requiring such performance has been defined as “ultra-reliable and low-latency communications” (URLLC). See, “Study on New Radio Access Technology; Radio Interface Protocol Aspects” (3GPP TR 38.804, v0.4.0). Note that URLLC traffic is applicable in a wide range of use cases not limited to the surgical/machinery examples set out above.
In current legacy schedulers, in order to ensure a reliable service, data units transmitted by a sender to a receiver over the radio interface are normally followed by an acknowledgement message sent by the receiver to the sender. If a positive acknowledgement message is not received for the transmitted data unit (i.e. the acknowledgement message is negative, or no acknowledgement message is received within a defined time window), then a re-transmission of the data unit may take place. This process is known as automatic repeat request (ARQ).
This scheme or variations of it are used in LTE in the following cases:                In the Medium Access Control (MAC) protocol, the sender may re-transmit a MAC protocol data unit (PDU), also referred to as a transport block, in case of no successful acknowledgement from the receiver.        In the Radio Link Control (RLC) protocol, the sender may re-transmit an RLC PDU in case it has received a status report from the receiver indicating negative acknowledgement for the indicated RLC PDU.        
In both of these cases, there will be some latency while the sender is waiting for the acknowledgement from the receiver before it can re-transmit the data unit, if necessary, and this will introduce additional latency in the overall end-to-end round-trip-time (RTT).